Introduction

Chronic rhinosinusitis with nasal polyps (CRSwNP) is a major concern with a prevalence of 2–4% in western countries [1]. Symptoms of loss of smell, nasal congestion and/or nasal obstruction, and rhinorrhea have a significant impact on social and physical quality of life. They also cause a significant economic burden due to absenteeism and cost of treatment [2]. CRSwNP is associated with bronchial hyperreactivity or asthma in 31 to 42% of cases [3]. Non-steroidal anti-inflammatory drug-exacerbated respiratory disease (N-ERD) is also reported in 9.7% of patients [4].

Many pathophysiological mechanisms involving both innate and adaptive immune systems have been described to explain the persistent inflammation within the nasal mucosa [5]. It is believed that the disease originates at the epithelial barrier, where it is triggered by a specific sinonasal microbiotic environment, under certain genetic predisposition and inflammatory conditions [6, 7]. The immune response itself then promotes inadequate wound healing of the epithelium, triggering abnormal tissue remodeling which leads to polyp formation [8, 9]. The intricate mechanistic hypotheses reported in the literature underline the important heterogeneity of CRS. The disease has multiple clinical presentations and biological features, with variable response to treatment (as demonstrated by new biologics targeting specific cytokines), shedding light on the probable inaccuracy of a single physiopathological model to explain chronic sinonasal inflammation [10].

Over the last few years, several studies have been conducted to delineate clinical and biological phenotypes of CRSwNP in relation with biomarker expression. Inflammatory endotypes have been described, with Caucasian population prone to lymphocytic T helper type 2 (Th2) inflammatory pathway (so called T2 inflammation) with an eosinophilic-skewed response and Asian population prone to Th1 or Th17 inflammatory pathways (T1 and T17 inflammation) [10]. The differences in biological patterns when considering ethnic origin may account for genetic factors, as US-second-generation Asians with CRS exhibit more non-eosinophilic patterns [11]. Environmental factors may also play a role in the inflammatory pattern, as seen in Thai patients with CRS symptoms who showed more eosinophilic patterns associated with more staphylococcus aureus colonization [12].

CRSwNP in the Caucasian population is biologically associated with overexpression of relevant T2 cytokines (Interleukin (IL)-4, IL-5, IL-13), leading to eosinophil (EO) recruitment and activation within nasal polyps. This profile is clinically associated with more frequent failure of steroid treatment, increase need for surgery to control the disease, and more frequent concomitant asthma compared with T1/T17 “neutrophilic-associated” CRSwNP [1]. The concept of “Eosinophilic Chronic Rhinosinusitis with or without Nasal Polyps” needs to be refined to account for suspected different underlying biological pathways. A minor part of Caucasian CRSwNP can exhibit a relatively EO deficient inflammatory infiltrate with a predominant T2 cytokine profile. In that perspective we should also consider priming and activation status of EOs in the bloodstream and nasal mucosa to better describe CRSwNP endotypes beyond a given cytokine cluster of expression.

In western countries, the EO mediator is always regarded as a major actor of CRSwNP inflammation. EOs belong to the innate immune system. They account for a small proportion of blood leukocytes (< 5%) and are mostly tissue-resident cells. The two main roles of EOs have long been supposed to be host defense against helminths and allergic inflammation maintenance. Several studies have brought new insights in the field of EO-associated diseases and biology. They allow us to reconsider EOs as pluripotent actors in many inflammatory diseases and physiologic conditions [13] (Fig. 1).

Fig. 1
figure 1

Eosinophil effectors functions. Eosinophil activation is mediated by a wide variety of surface receptors that respond to diverse stimuli. Upon activation, eosinophils promote host protection via direct effects on pathogens or tumoral tissues, immune responses by modulation of lymphocyte and dendritic cell function, and airway inflammation via tissue damage, remodeling, and mucous hypersecretion. They are also involved in homeostatic functions (adipose, thymic and mammary gland development)

Full size image

Our review aims to describe EO involvement in CRSwNP when considering cell maturation, nasal homing, cytolytic properties. and interaction with the immune system. Each step leads to a better understanding of new treatment options offered by biologics in nasal polyposis.

From Bone Marrow to Nasal Polyps

EO Development and Maturation

EO lineage commitment has been reviewed by Mack et al. in 2020 [14]. Many available results come from mice studies and still need to be confirmed in human models. Herein, the review will focus on data obtained from human EO cell-line studies. Like other leukocytes, EOs are generated in the bone marrow from eosinophil progenitors (EoPs). These progenitors seem to derive from a common myeloid precursor (CMP), although EO might possibly derive from earlier progenitors [15]. The whole process depends on a precisely tuned combination, in both time and quantity, of non-specific transcription factors, including GATA-binding protein transcription factors (whose main factor is GATA-1, C/EBP proteins, and PU.1) [14] (Fig. 2). According to McNagny et al.’s description of EO differentiation from other cell-lineages via subtle modifications in transcription factors, some authors speculate that different inflammatory environments could modify transcriptomic pathways, and consequently eosinophilopoiesis at the bone marrow level [16]. Along with their role in regulating EO differentiation, GATA-1, C/EBP, and PU.1 are involved in the transcription of several important EO proteins such as Major Basic Protein-1 (MBP-1), Eosinophil Peroxidase (EPO), or C-C Motif Chemokine Receptor 3 (CCR3) [17,18,19].

Fig. 2
figure 2

Eosinophil journey from bone marrow maturation to nasal migration. Eosinophil (EO) develops in the bone marrow. Transcription factor (such as GATA-1, C/EBP proteins and PU.1) and cytokines (such as interleukin (IL)-5, IL-3 and GM-CSF) are essential for its differentiation from CD34+  common myeloid precursor (CMP) to IL-5Ra+ eosinophil progenitors (EoP) and mature EO. Once mature, IL-5 controls the EO migration from the bone marrow to the blood. EO chemotaxis in nasal mucosa is mediated by the combination of the local production of cytokines and chemokines controlled by alarmins (thymic stromal lymphopoietin (TSLP), IL-25 and IL-33) secreted by epithelial cells after damage associated molecular pattern (DAMP) release under epithelial barrier dysfunction. The expression of homing molecules (Very Late Antigen (VLA)-4, CD11a) and of CD44 on the EO surface is required to allow interaction with their counter-ligands expressed on inflamed endothelium of the nasal mucosa. C-C Motif Chemokine Receptor 3 (CCR3)-binding eotaxins 1, 2, and 3 are also produced by epithelial cells and fibroblasts at baseline and under lymphocytic T helper type 2 (Th2) stimulation. Regulated on Activation, Normal T Cell Expressed and Secreted (RANTES) and dysregulated prostaglandin D2 (PGD2) secretion are involved in the process of EO directional migration into the extravascular compartment. IL-4 and IL-13 released by Th2 cells and type 2 innate lymphoid cells (ILC2) contribute to EO chemotaxis. EO exhibit prolonged survival in nasal mucosa due to their protection from cell death by locally produced cytokines such as IL-5 and GM-CSF. Once activated, EO are able to express cytokines and cytotoxic granules proteins. These cytotoxic proteins (Major Basic Protein-1 (MBP), Eosinophil Peroxidase (EPX), Eosinophil Cationic Protein (ECP), Eosinophil Derived Neurotoxin (EDN)) are involved in nasal mucosa damage and tissue remodeling. Eosinophil progenitors (EoPs) have been identified in CRSwNP within nasal mucosa and might subsequently mature in the presence of a favorable local microenvironment

Full size image

EoPs can be defined by a CD34+ IL-5 receptor alpha (IL-5Ra)+ CMP phenotype as these cells can only differentiate into EOs [15]. The interaction of IL-5Ra with its ligand on the EoPs membrane plays a pivotal role in EO maturation, proliferation, and survival properties. Activation processes through IL-5Ra is also essential to promote massive EO migration within tissues under inflammatory circumstances, such as CRSwNP [20].

IL-5 production is supported by Th2-cytokine secreting cells, mainly innate lymphoid cell type 2 (ILC2) and Th2 lymphocytes, but also at the mucosal level by mature EOs themselves leading to a self-stimulating survival loop under inflammatory circumstances [21, 22]. IL-5 independent activation pathways have been described in IL-5 deficient mice with almost normal numbers of circulating and tissue-resident EOs, possibly via a Macrophage Inflammatory Protein-1 alpha (MIP-1α or CCL3)-CCR1 interaction on EoPs [23, 24]. Less specific cytokines are also important in promoting EO differentiation, proliferation, and activation, i.e., IL-3 and GM-CSF whose receptors share a common β chain subunit with IL-5 and IL-4, leading to EO activation in an IL-5-dependant manner [24, 25]. Interestingly, some cytokines produced by damaged nasal epithelium seem to be involved in EoPs differentiation even before IL-5 exposure, as for example IL-33 [26].

Both mature EOs and CD34+ IL-5Ra+ EoPs are able to migrate from the bone marrow to other extramedullary sites via the bloodstream. EoPs could play a direct role in eosinophilic inflammation and have been identified in CRSwNP within nasal mucosa [27]. EoPs might subsequently mature in the presence of a favorable local microenvironment, as demonstrated ex vivo in allergic nasal mucosa after exposure to IL-5 [28].

Circulating Eosinophils in CRSwNP

Blood EO counts in CRSwNP patients with uncontrolled disease, or comorbid asthma stays most of the time within normal ranges, compared with healthy patients [29]. On the contrary, blood EO counts in some other eosinophilic disorders (helminth infection, hypereosinophilic syndrome) can easily ascend over 1.5 G/L, making eosinophil blood count a poor phenotyping marker in eosinophilic chronic rhinosinusitis diagnosis in comparison. However, some authors referred to absolute blood EO count as a possible prognostic factor of disease control, as they hypothesized that it would correlate to a lower number of tissue EO, which has yet to be demonstrated [30].

Beyond these considerations, other specific approaches to blood EO “activated status” could be more relevant to describe patient’s inflammatory status. Circulating EOs can exhibit an activated phenotype as demonstrated by Dupuch and al., with a significant increase of EOs expressing activation markers CD49d (Very Late Antigen-4 (VLA-4)), CCR3, and CD25 (IL-2Rα) in patients with CRSwNP, and a higher oxidative metabolism compared with control patients [31]. Similarly, the activation status of blood EOs based on CD69 and CD44 membrane expression cytokines can be correlated to concomitant asthma in CRSwNP [29]. In a prospective study including patients with corticosteroid resistant CRSwNP, we showed that IL-5Ra was less expressed on EO membrane in nasal polyps of asthmatic patients. No difference was observed for IL-5Ra expression on EOs collected from blood samples of the two different patient populations [29].

Eosinophil Tissue-Recruiting Modalities in CRSwNP

EOs are mainly tissue-resident cells with an estimated time spent in blood circulation of one day and a tissue lifetime believed to last up to several days [32].

After bone marrow maturation and their release into the blood stream, EOs are recruited at the level of the nasal mucosa where they start to exert their multiple functions (Fig. 2). Even though numerous studies tried to assess for a diagnostic mucosal EO threshold, there is no specific cut-off in quantitative tissue eosinophilia to reliably differentiate between eosinophilic CRS and non-eosinophilic CRS. Even if there is not yet an international consensus on the measure, a generally accepted 10% ratio of EO in nasal secretions seems to be in favor of eosinophilic CRS. A meta-analysis suggested that a count of 55 EOs or more per high-power field (× 400 magnification) on histologic specimens was specific enough to define eosinophilic CRS. This result still has to be reinforced by future studies as it appeared that included studies displayed a wide range of cut-off values (from 5 to 120) [33]. A higher tissue EO count could be associated with a more aggressive disease and a poorer disease outcome [34]. In a prospective study including 36 consecutive patients who underwent endoscopic sinus surgery for CRSwNP resistant to optimal medical treatment, we showed that mucosal eosinophilia was a marker of concomitant respiratory disease [35].

The expression of homing molecules (VLA-4, CD11a (Integrin αL), and of CD44 (Homing Cell Adhesion Molecule (H-CAM)) on the EO surface is required to allow for adherence onto endothelial cells and diapedesis into target tissues. This expression can be further modulated to maintain EOs within mucosal tissues after transendothelial migration [29]. We also showed that the expression of β integrins (VLA-4, CD11a) by EO purified from blood samples and nasal polyps of patients with steroid resistant CRSwNP was reduced both in asthmatic and non-asthmatic patients [29]. EO chemotaxis in nasal mucosa is mediated by a combination of locally produced cytokines and chemokines controlled by alarmins, released by triggered epithelial cells. Thymic Stromal Lymphopoietin (TSLP), IL-25, and IL-33 are produced after damage associated molecular pattern (DAMP) release [36, 37]. CCR3-binding eotaxins 1, 2, and 3 (C-C Motif Chemokine Ligand (CCL)11, CCL24, CCL26) are also produced by epithelial cells and fibroblasts at baseline and under Th2 inflammatory stimulation [38, 39]. Regulated on Activation, Normal T Cell Expressed and Secreted (RANTES/CCL5) and dysregulated prostaglandin secretion with increased chemotactic Prostaglandin D2 (PGD2) (via Chemoattractant-homologous Receptor expressed on TH2 cells (CRTH2) expressed by EOs) and fewer anti-inflammatory Prostaglandin E2 (PGE2) are also involved in this process and contribute to upregulate the local inflammatory processes [40, 41]. Along with their own chemotactic abilities, these chemokines are able to promote a pro-Th2 inflammatory environment, through ILC2 activation and Th2 lymphocytes recruitment [36]. Subsequent production of IL-4, IL-13, and IL-5 contributes to EO attraction via the upregulation of endothelial adhesion markers [36, 42, 43].

EO tissue migration can be described as a stepwise procedure. Tethering and rolling are mediated by selectins, whereas firm adhesion and extravasation are mediated by integrins [44]. Only few studies described the role of adhesion molecules during human EO transendothelial migration in CRSwNP. Some authors showed that EO mucosal infiltration was related to the increased expression of Vascular Cell Adhesion Molecule-1 (VCAM-1) in nasal polyps [45, 46]. On the contrary Symon et al. showed no increase in the expression of VCAM-1 in nasal polyps but an overexpression of InterCellular Adhesion Molecule-1 (ICAM-1), P-selectin, and E-selectin [47]. Patel et al. found that EO tethering to IL-4 activated endothelial cells required P-selectin and VCAM-1 [48]. More recently, Eweiss et al. found that VCAM-1 expression was increased on nasal polyps vessels compared with the mucosal surface of healthy patients. This upregulated expression of VCAM-1 did not statistically correlate with the importance of EO mucosal infiltration and was comparable between nasal polyp tissue and inferior turbinate mucosa [49].

Eosinophil Behavior in CRSwNP Mucosal Settings

EOs exhibit many roles in CRSwNP pathophysiology: cytotoxic function, immunoregulation, and chemotaxis (Fig. 1). Some are well-described, others are still in debate. Many hypotheses supporting EO pivotal role in CRSwNP are based on the analysis of biomarkers expression from total nasal polyp homogenates. Mechanistic assumptions may be incomplete and need to be considered with caution.

EOs can secrete multiple well-known cytotoxic cationic proteins (Major Basic Protein-1(MBP), Eosinophil Peroxidase (EPX), Eosinophil Cationic Protein (ECP), Eosinophil Derived Neurotoxin (EDN)) and reactive oxygen species (ROS) and more than 30 cytokines, chemokines, and growth factors involved in immunoregulation (Th1 cytokines (Interferon gamma, IL-12), Th2 (IL-5, IL-4, IL-13, IL-9) but also tumor necrosis factor alpha (TNFα), IL-6, IL-10, proliferation-inducing ligand (APRIL), tumor necrosis factor (TGF)-β, and vascular endothelial growth factor (VEGF)) [50].

All of these molecules are stored within large, dense granules called “specific granules,” distinct from smaller “primary granules” containing Charcot-Leyden crystal protein (also known as Galectin-10), lipid bodies producing eicosanoids, including cysteinyl leukotrienes, prostaglandins, and thromboxanes [51]. Storages within granules, along with transcription of preformed cytoplasmic mRNA, enable pro or anti-inflammatory responses, with a fast release of preformed mediators after EO stimulation by either cytokines, immunoglobulins, platelet activation factor, or pattern recognition receptors [52,53,54,55]. Several ways of granule releasing have already been described [56] (Fig. 3):

  • Compound exocytosis is a common cell mechanism leading to the fusion of secretory vesicles with the membrane and exteriorization of the granule contents.

  • Cytolysis, with liberation of intact granules, able to release their cytotoxic activities inside the inflamed tissue long after the death of the EO.

  • Piecemeal degranulation (PMD), an intriguing mechanism of specific-granule-content delivery, with differential secretion within EO granules, therefore leading to a precise cytokine-specific secretion despite heterogeneous granule contents.

  • “ETosis,” for extracellular DNA traps, is a recently described mechanism during which EOs release DNA “nets” associated with granule contents, either in an eosinophil-living mechanism involving mitochondrial DNA or a non-apoptotic cell-death mechanism involving nuclear DNA [57, 58]. ETosis can be triggered by lipopolysaccharide (LPS), complement factor 5a, eotaxin, or TSLP exposure and seem to be involved in the mechanism of antibacterial defense but can also promote inflammatory maintenance, acting as a DAMP [59]. ETosis could be compared with formerly described cytolytic EOs, found in tissue sections, and associated with free functional granules.

Fig. 3
figure 3

Processes of eosinophil granules secretions. Resting eosinophil shows the cytoplasm packed with granules full of contents with typical morphology. Each granule has a central dense core surrounded by a delimiting membrane a. Eosinophils may secrete their granule proteins by classic exocytosis (individual granule fusion with the plasma membrane and release of the total granule content) or compound exocytosis (intracellular granule–granule fusion before extracellular release) b, piecemeal degranulation (vesicular transport of small packets of materials from the secretory granules to the cell surface) c, ETosis (intact granules entrapped in web-like extracellular DNA traps) d, and/or cytolysis (extracellular deposition of intact granules upon cell lysis) e. More than one process can be involved in inflammatory responses. Storages of proteins (enzymes, cytokines, growth factors, and chemokines) within granules, along with transcription of preformed cytoplasmic mRNA, enable pro or anti-inflammatory responses, with a fast release of preformed mediators after EO stimulation

Full size image

Degranulating patterns are mostly CD63-associated regulated mechanisms, as EO stimulation with eotaxin1/CCL11 seems to promote PMD, whereas stimulation with TNFα seems to induce preferentially exocytosis [60].

As EOs mainly exhibit secreting functions, degranulation was studied in several studies, on small populations of patients with CRS. Armengot et al. studied EO degranulation on 582 EOs from nasal polyps of 36 patients using electronic transmission microscopy [61]. They showed that 30% of EO were inactive; 42% exhibited signs of piecemeal degranulation. Cytolysis involved 27% of EOs and could eventually be related to ETosis phenomenon. In that study, apoptosis was negligible (0.3% of EOs). The degranulation process can be initiated in IL-3 or IL-5 primed EOs (EOs that have been in contact with IL-3 and IL-5) after interacting with IgG, IgA and IgE antibodies, CCR3-binding eotaxins, RANTES or Monocyte Chemoattractant Protein-4 (MCP-4), complement components, or fibrinogen [53, 62,63,64,65]. However, as most of those results derive from in vitro models, they must be considered with caution as they do not reflect in vivo interactions within intricate inflammatory environments. So far, no study focused on degranulation patterns frequency and their relationship with disease severity.

Extracellular release of cytotoxic proteins is suspected to play a role in nasal mucosa remodeling, as blood and tissue overexpression of specific cytotoxic proteins from activated EO is associated with mucosal disease severity [66]. Tsuda et al. described that in vitro exposure of epithelial cells to EDN affected the epithelium to mesenchymal transition pathway with overexpression of Matrix MetalloProteinase-9 (MMP-9) mRNA. As EDN and MMP-9 are abundant in nasal polyps, they suggest that EDN could play a role in CRSwNP airway remodeling [67]. This hypothesis is concordant with a previous study analyzing in vitro impact of ECP on bronchial epithelial cell remodeling where overexpression of endothelin-1, TGF-alpha, TGF-beta1, platelet-derived growth factor (PDGF)-beta, epidermal growth factor (EGF) receptor, MMP-9, fibronectin, and tenascin were observed [68]. EDN also seems to act as an alarmin, attracting dendritic cells and maintaining a pro-Th2 environment [69].

Eosinophil extracellular trap formation or ETosis is a recently described EO function. It was observed in nasal secretions, partly explaining their viscosity in CRSwNP due to their high DNA and protein concentrations, and trapping pathogens to facilitate their clearance [57]. ETosis can be activated in primed EOs by TSLP, complement factor 5a, LPS, and eotaxins [70, 71]. ETosis association with staphylococcus aureus antigens is interesting as staphylococcal-associated IgE production has been widely described as a keystone for CRSwNP onset [72]. ETosis was observed in 8.8 ± 4.8% of nasal EOs and was associated with increased expression of both IL-5 and periostin along with higher levels of staphylococcus aureus colonization [73]. Ex vivo procedures with exposure to staphylococcus aureus at the epithelial barrier showed a significant increase in immediate ETosis. The same results were observed with staphylococcus epidermidis when incubated with TSLP [70]. As a consequence, ETosis seems to be associated with disease severity in both eosinophilic chronic rhinosinusitis and non-eosinophilic chronic rhinosinusitis [74].

Along with their secretory abilities, EOs play a role in chemotaxis [44]. They are able to locally attract other cells involved in the mucosal inflammatory response, including Th2 lymphocytes, neutrophils, dendritic cells or monocytes/macrophages via MBP-1, cationic proteins, or chemokines releasing [21, 69, 75, 76]. EO-produced CCL23 could also attract macrophages, monocytes, and dendritic cells locally [21]. EOs can also play a minor antigen-presenting role, can contribute to prolonged B cell survival, and can support efficient IgA class-switching via APRIL secretion [77, 78].

EOs are also involved in tissue homeostasis and regeneration via regulatory and anti-inflammatory mechanisms [79]. They are consistently found in tissues with high epithelial cell turnover and major stem cell activity (small intestine, uterus, bone marrow, adipose tissue) [80]. The Local Immunity And/or Remodeling Repair (LIAR) hypothesis described by Lee et al. accounts for the nuanced EO role in both healthy and diseased patients. Instead of EOs exclusively mediating innate host defense, the authors suggested that accumulating tissue EOs become regulators of LIAR in both healthy and pathologic conditions [81]. Such hypotheses could be supported by variations in the local cytokine environment, differential effects on inflammatory and anti-inflammatory processes, or heterogeneous EO subpopulations [82, 83].

From Pathophysiology to Biologic Therapies in CRSwNP

Over the last 15 years, biologic therapies targeting EOs markers and T2 inflammation pathways have been developed to target life-threatening eosinophilic disorders, like hypereosinophilic syndromes or severe asthma. Consisting of monoclonal antibodies (mAb), these molecules have also been thoroughly evaluated in other eosinophilic disorders such as atopic dermatitis, eosinophilic esophagitis, and subsequently in eosinophilic CRSwNP [84] (Fig. 4). Pro-T2 inflammatory profiles have been associated with medical and surgical failure in CRSwNP [85]. Biologics offer new therapeutic options when both steroids and endoscopic sinus surgery are insufficient to control disease severity and improve quality of life in the long term.

Fig. 4
figure 4

Mechanism of action of biologic therapies for treatment of CRSwNP. Some agents target immunoglobulins (Ig) (omalizumab), cytokines (mepolizumab, reslizumab), or chemokines (bertilimumab). Others target lymphocytic T helper type 2 (Th2), type 2 innate lymphoid cells (ILC2), or eosinophil (EO) surface receptors (respectively dupilumab, benralizumab, and anti-Siglec-8 mAb). Omalizumab binds to soluble IgE produced by B cell and blocks IgE interaction with mast cell and basophil via the high-affinity receptor for the Fc region of immunoglobulin E (FCɛRI). Mepolizumab and reslizumab target soluble interleukin (IL)-5, whereas benralizumab targets the IL-5 receptor alpha chain (IL-5Ra) reducing EO activation and survival. Dupilumab targets the IL-4 receptor alpha chain (IL-4R) and blocks the downstream signaling of IL-4 and IL-13 (i.e., release of eotaxin and thymic stromal lymphopoietin (TSLP) by the nasal epithelium, skewed-Th2 differentiation of naive helper T cell (Th0), activation and differentiation of B cells into plasma cells, tuning of ILC2). Anti-Siglec-8 mAb induce antibody-dependent cellular cytotoxicity activity (ADCC) leading to EO depletion. Tezepelumab binds to TSLP consequently reducing EO chemotaxis in nasal mucosa and type 2 cytokines release. Bertilimumab blocks eosinophil chemotaxis via eotaxin1/CCL11 pathway

Full size image

Anti-immunoglobulin E

Anti-IgE mAb (omalizumab) was first used in asthmatic patients, but quickly assessed in CRSwNP as a high production of IgE can be found in nasal polyps, independently of associated allergy [86]. The therapeutic potential of omalizumab was reinforced after studies reported higher concentrations of staphylococcus aureus enterotoxins IgE in nasal polyps [87]. In small randomized, double-blinded, placebo-controlled studies, patients treated with anti-IgE mAb showed an improvement in CRS-related symptoms and endoscopic nasal polyp score (NPS), with decrease use of local steroid [88, 89]. Recently, 2 phase III trial studies including respectively 138 and 127 patients showed that omalizumab significantly improved endoscopic findings, clinical symptoms, and patient-reported outcomes in severe CRSwNP with resistance to optimal medical treatment [90]. No change in blood EO count was reported in these studies (Table 1).

Table 1 Biological findings of randomized, double-blinded, placebo-controlled studies with monoclonal antibody therapies in CRSwNP [88,89,90,91,92,93,94,95]
Full size table

Anti-Th2 Cytokines (IL-5, IL-4, IL-13)

Improvement in CRSwNP symptoms and/or radiologic score and/or nasal polyp size and/or surgery incidence has been described with anti-Th2 cytokines biotherapies [91,92,93,94,95]. The promising results of these double-blinded placebo controlled studies have been summed up in a review and a recent Cochrane analysis [96, 97].

Anti-IL-5 drugs have proved to be broadly effective with a decrease in blood EO count when patients are treated with rezlizumab and mepolizumab (Table 1). Immunomodulation of neutrophil-mediated inflammatory response was also suggested with decrease in serum IL-6, myeloperoxidase (MPO) and IL-1β levels (Table 1). The variable response to anti-IL-5 monotherapies in CRSwNP could be explained by a lower expression of IL-5R transmembrane protein in tissue eosinophils inside nasal polyps, possibly mediated by transcription modifications following GM-CSF and/or IL-3 and/or IL-5 stimulation [29, 98,99,100]. An increase in soluble IL-5Ra via a proteinase-mediated mechanism, or possibly via endocytic process of the whole receptor, has been advanced to explain these results [98, 101]. The findings of a multicentric randomized, double-blinded, placebo-controlled study with benralizumab (anti IL-5R) are still pending (OSTRO Clinical Trial Number: NCT03401229). Benralizumab is a humanized monoclonal antibody that targets the IL-5Ra chain. Unlike targeting soluble IL-5, this agent can induce antibody-dependent cellular cytotoxicity (ADCC) that can result in EO depletion.

Antibodies targeting the alpha subunit of IL-4 and IL-13 receptors show the most promising results in CRSwNP. Two multicentric randomized, double-blinded, placebo-controlled studies including respectively 276 and 447 patients demonstrated a significant improvement of CRS-related symptoms with a reduced polyp size and improved CT opacification [95]. A transient and non-significant increase in blood EO count was reported in both SINUS-24 and SINUS-52 studies. This is consistent with the hypothesis that dupilumab blocks EO nasal migration by inhibiting the production of EO-activating chemokines via a Th2 pathway (Table 1; Fig. 4).

Ongoing Development of Targeted Therapies

Other targeted molecules are engineered to block EO functions at different levels (activation phase, homing stage, and degranulation) (Table 2):

Table 2 Ongoing development of targeted therapies [102,103,104,105,106,107]
Full size table

A phase III clinical trial with oral anti-CCR3 treatment in patients with asthma or eosinophilic bronchitis demonstrated no clinical improvement despite high EO blood counts [102]. More recently, R321, a CCR3-binding inhibitory peptide scaffold with nanoparticles showed some efficiency in blocking EO recruitment in blood and airways, and decreasing airway hyperresponsiveness in a mouse eosinophilic asthma model [103].

Bertilimumab blocking Eotaxin1/CCL11 pathway has been evaluated in a phase IIa clinical trial on patients with pemphigus or pemphigoid auto-immune skin diseases associated with severe skin hypereosinophilia (Clinical Trial Number: NCT02226146). Results were promising with a substantial improvement in disease activity after only 3 doses, pushing for further studies to clarify the exact impact of bertilimumab.

Siglec-8 is a surface receptor expressed only on mature EOs, basophils, and mast-cells. Siglec-8 activation by its sialoglycan ligand induces EO apoptosis, thus making Siglec-8 an interesting target in eosinophil-mediated diseases. AK002/Antolimab, an anti-Siglec 8 antibody, is capable of inducing apoptosis and NK cell-dependent antibody-derived cytotoxicity on IL5-primed EOs in the bloodstream as well as on EOs collected from human lungs [104]. A multicenter phase II randomized, double-blinded, placebo-controlled trial evaluating this treatment in eosinophilic gastritis showed histologic and symptomatic improvements [105].

Anti-GM CSF monoclonal antibody KB003 was tested on patients with uncontrolled asthma. The primary outcome was Forced Expiratory Volume1 (FEV1) after 6 months of treatment, and secondary outcomes included exacerbation rate, general control of asthma, and safety of use. No statistical differences were found between the treatment group and the placebo group, except for patients with baseline blood EO count ≥ 0.3 G/L in the KB003 group [106].

Several clinical trials using blocking monoclonal antibodies against alarmins are in being conducted in refractory asthma but also in CRSwNP. Tezepelumab, a monoclonal antibody targeting TSLP, showed a decrease in asthma exacerbations compared with placebo in patients with uncontrolled asthma in a phase II clinical trial [107].

Perspectives

Describing the specific roles of EOs in CRSwNP is a challenging task, as nasal polyps are complex and heterogeneous inflammatory microenvironments harboring multiple inflammatory cells with intricate functions. Within the nasal mucosa, EOs can express either innate cytotoxic functions or immunoregulatory functions through different secretory pathways. The exact triggers of EO activation within the nasal mucosa and their impact on disease severity remain unclear. This may in part explain some heterogeneous results observed in recent clinical trials, as we can presume that EO role is incompletely understood. Another point of complexity is the intertwined inflammatory pathways involved in inflammatory mucosal diseases. EOs are one of the actors of a complex process but it would be a mistake to picture them as the major player of the equation. From a clinical standpoint, it seems that multi-treatment approaches using biologics hold the most promising candidates to effectively target all aspects of the inflammatory response in patients with CRSwNP.

From a research and development perspective, a more functional approach to the behavior of EOs in the bloodstream and in tissues would be of great help to delineate specific target mechanisms. Many research teams now try to conduct large proteomic and transcriptomic analyses off nasal polyps in order to extrapolate EO functions in CRSwNP. Meanwhile, EO isolation and purification from tissues are still not easy to achieve. In vivo studies using animal models are only partially adequate, as EOs can exhibit functional differences between species. Finally, in vivo analysis of human EOs requires large blood samples and fresh nasal mucosal tissue, which could be considered invasive from an ethical standpoint.

Development of new in vivo study models, such as nasal mucosa organoids, could be important tools to better understand the role of EOs in CRSwNP. This review calls for collaborative thinking in the fields of proteomic analysis, cellular studies, and new in vivo research models to improve our understanding of this complex but fascinating disease.